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Diversity of Mechanisms for Boron Toxicity in Mammals
Published in Debasis Bagchi, Manashi Bagchi, Metal Toxicology Handbook, 2020
Diana Rodríguez-Vera, Antonio Abad-García, Mónica Barrón-González, Julia J. Segura-Uribe, Eunice D. Farfán-García, Marvin A. Soriano-Ursúa
Simple boronic acids have not shown to induce fatal toxicity in humans. However, the limitations for their use as therapeutic agents also have limited the studies in humans. Some molecules, including boronic or structurally related moieties, such as benzoxaboroles or the proteasome inhibitors (Table 26.3), have shown similar toxicity as their free-boron analogs. Boronic acid compounds have shown effects as potent enzyme inhibitors, as boron neutron capture agents in cancer therapy, and as antibody mimics that also recognize important saccharides. The interaction of the boronic moiety with diols seems to play a key role in these mechanisms. Some examples of these BCCs are boronic acid, bonzoxaborole compounds, boron-containing anticoagulants, and phenylboronic acid derivatives, which have been developed and considered for different clinical treatments with no available information about toxicity (Priestley et al. 2002; Baker et al. 2006, 2009; Venkatraman et al. 2009). Also, boronic acid inhibitors of DPP4 have contributed substantially to the efficacy of post-meal blood glucose levels (Baker et al. 2009). The ability of these compounds to interact with diols could involve a high affinity of enzymatic or non-enzymatic systems triggering the molecular mechanism of toxicity.
Recent Advances in Boron-Based Flame Retardants
Published in Yuan Hu, Xin Wang, Flame Retardant Polymeric Materials, 2019
Unlike the B–O–C bond in boric acid ester, boronic acids with the B–C bond are hydrolytically stable. There have been many publications on the use of boronic acid derivative as flame retardants. They are known to release water on thermolysis, thereby leading to the formation of boroxines or boronic anhydrides. For example, chemical modification of PS by introducing boronic acid functionality (–B(OH)2) to predominately the para-position improves fire retardancy. It was reported that the boronated PS is significantly less flammable than the unmodified PS. The virgin PS has a LOI of 18.3% vs. 25.3% for the boronated PS (degree of substitution 9.2%). The char yield also increased from <1% to 7% at 600°C. It was postulated that the boronic acid groups are active in the condensed phase by assisting intermolecular crosslinking, forming the 6-membered boroxine ring, and promoting the formation of a protective char layer (Armitage et al. 1996). It was reported that 10 mole% of a boronic acid derivative (two boronic acid/pinacol moieties attached to dimethyl terephthalate) can reduce total heat release reduction by 20% in a flexible polyurethane formulation (Benin et al. 2014).
Metal-Catalyzed Condensation Polymerization
Published in Samir H. Chikkali, Metal-Catalyzed Polymerization, 2017
Suzuki and Stille polymerization involves the use of toxic starting materials such as boronic acid and stannanes. The boronic acid or stannane coupling partners need multiple-step synthesis, which limits their direct use. To circumvent this problem, a metal-catalyzed coupling of aromatic C–Br bonds with aromatic C–H bonds called direct (hetero)arylation polymerization (DHAP) or direct arylation polymerization (DArP) was developed.38,39 This method affords higher yields as well as higher molecular weights compared to conventional polymerizations and even unfunctionalized monomers can be used. The conjugated polymers produced show properties comparable to analogous polymers synthesized by Suzuki or Stille coupling. The mechanism follows a base-assisted, concerted metalation–deprotonation pathway for most heterocycles, however electrophilic aromatic substitution and Heck-type coupling may occur depending on the substrate leading to certain defects in the resultant polymer structure. Typically, carboxylate or carbonate anions are used as additives although the reaction also works without these additives. Under carboxylate-mediated conditions, after the oxidative addition of the carbon–halogen bond to Pd(0) catalyst the halogen ligand is exchanged for the carboxylate anion. Thiophene substrate is deprotonated by carboxylate ligand while simultaneously forming a metal–carbon bond. The phosphine ligands, or the solvent, can recoordinate to the metal center or the carboxylate group can remain coordinated throughout the entire process. The product is then obtained by reductive elimination process. A major drawback of this process is the lack of C–H bond selectivity, particularly for thiophene substrates, which can result in cross-linked material during polymerization reactions.
Palladium catalyzed cross-coupling of 3-methylthiophene-2-carbonyl chloride with aryl/het-aryl boronic acids: a convenient method for synthesis of thienyl ketones
Published in Journal of Sulfur Chemistry, 2022
Komal Rizwan, Idris Karakaya, Muhammad Zubair, Nasir Rasool
To find the effect of base on conversion of starting material to product, different bases K2CO3, Na2CO3, Cs2CO3, K3PO4 were included in the screening and showed comparable product to internal standard (4,4'-di-tert-butyl-1,1'-biphenyl) ratios, with Cs2CO3 providing the highest conversion to product (3a) (Table 3). To find the optimal amount of base, the reaction was carried out with different base loadings, and 1.5 eq of Cs2CO3 provided the highest conversion to cross coupled product (3a) (Table 4) in presence of Pd(PPh3)4 (5 mol%), at 50 °C for 18 h in toluene (0.1, 5 mL), under argon. Unfortunately, homocoupling of boronic acids were observed in the reaction mixtures. Attempts to augment reactivity under open air were made but unfortunately high amount of starting material was recovered however, effective and reliable yields were obtained under argon atmosphere.
Flow reactor synthesis of unsymmetrically substituted p-terphenyls using sequentially selective Suzuki cross-coupling protocols
Published in Green Chemistry Letters and Reviews, 2019
Shahid A. Kazi, Eva M. Campi, Milton T. W. Hearn
As highlighted in Table 3, the developed protocol resulted in an efficient in-flow ligand-less double Suzuki coupling of 1,4-dibromo-2-nitrobenzene (1) with up to 78% yield of the double coupling product was obtained. Depending on the reactivity of the second boronic acid, significant amounts (ca. 20%–25%) of products (4) and (5) derived from the initially formed mono-coupled product (3) could be isolated. However, implementation of simple rate enhancement steps at the stage of the second coupling procedure, such as an increase in temperature or longer residence time as practical process intensification steps, could shift the reaction towards the double coupling product (7). Previous double Suzuki coupling carried out under batch conditions led to higher yields of (7), with only mono-coupled product (3) isolated (in reactions with (6c) and (6d)) (40). These batch reactions, however, needed to be carried out under quite different conditions to achieve site selectivity, namely 0°C for the initial coupling, 25°C for the second coupling and THF:H2O (1:1) as the solvent.